1. Field of the Invention
The present invention generally relates to an image reading apparatus and an image forming apparatus. More particularly, the present invention relates to an image reading apparatus including a contact glass on which a document is placed; an imaging device configured to image a document image based on light reflected from the document through the contact glass; plural line image sensors arranged along a main scanning direction at positions where the document image is imaged and configured so that end portions of the line image sensors adjacent to each other similarly read same image data imaged at an overlapping area, and an image data joining device configured to join the image data at the overlapping area similarly read by the end portions of the plural line image sensors to form continuous image data, and an image forming apparatus having such an image reading apparatus.
2. Description of the Related Art
There are some different sizes of sheets to be read by an image reading apparatus, and some image reading apparatuses are capable of reading A0 size documents. The image width of the A0 size is typically about 840 mm. When the whole A0 size document is read by a single CCD line image sensor under 600 dpi resolution, approximately 20,000 pixels image sensors are required to be arranged on the single line image sensor. However, a line sensor having such a large number of pixels is difficult to be manufactured with current technology at reasonable costs, namely the manufacturing cost of such a line sensor becomes high.
Therefore, due to the current limitation of the number of pixels on a single line image sensor, in a document reading apparatus capable of reading an A0 size and/or A1 size document, there has been used a method of reading an image, in which plural CCD line sensors are arranged partially overlapping each other in the main scanning direction (see, for example, Patent Documents 1 and 2).
However, in a case where this method is employed, even when the overlapping portion of the line sensors is strictly adjusted in its manufacturing process, the overlapping portion may become out of alignment due to vibration during transportation, out of alignment of its main body of the system during its mounting, or the like. Once the overlapping portion is out of alignment, the out of alignment of the overlapping portion of the image reading apparatus remains and may not be easily restored. Further, the conjugation length of the image forming optical system of the image reading apparatus typically requires approximately 500 mm to 800 mm, and the reduction ratio of the optical system becomes approximately 1/15. Therefore, if the CCD line sensor is misaligned by only 10 μm, the image on the surface of a document may be misaligned by as much as 150 μm.
Further, such misalignment in the line image sensors may be generated not only in its manufacturing or mounting process but also by thermal expansion due to the temperature change in the image reading apparatus during operation. Such misalignment (position shift) may be caused in both main scanning direction and sub scanning direction.
To overcome the problem, Patent Document 3 discloses a technique in which a marking is formed at an overlapping reading area read by the CCD line sensors, and the whole image forming optical system is configured to be mechanically shifted in the sub scanning direction to detect the relative position of each optical system, and a sub scanning position is adjusted so that the position shift is minimized.    Patent Document 1: Japanese Patent Application Publication No.: S62-101170    Patent Document 2: Japanese Patent Application Publication No.: S56-126373    Patent Document 3: Japanese Patent Application Publication No.: 2000-358140
However, when a technique such as that proposed in Patent Document 3 is employed, there may arise a problem that the line image sensors are required to be mechanically shifted and guide rods and cams are required to be provided to guide the line image sensors, thereby increasing the cost and impairing the durability of the image reading apparatus. In addition, even when the overlapping portion is well adjusted, the parallelism between the optical systems may be degraded by the position shift of the optical system when, for example, the guide rods become loose. Further, in order to detect the position shift, the line image sensors are required to be moved back and forth to detect the peak position of the line image sensors. Therefore, it takes time to complete the adjustment and a long wait time is required.
In the above configuration, however, a marking for position adjustment may be placed at a position on the contact glass, so that a shifted amount of the marking position detected by each line image sensor is obtained. Based on the obtained shifted amount, data may be shifted to correct the position shift. Unfortunately, this method may not be appropriate because the marking position, i.e., the center portion of the read image, may be shaded and a signal of that portion may not be well obtained. To avoid this problem, a marking may be placed on a member other than the contact glass, and the marking may be configured to be moved outside the image reading area when an image is being read. However, even in this method, there may arise problems; for example, the marking position may be shifted when the movement of the marking is repeated, the marking position may be separated from a document surface level, a surface of document may become dirty, and the cost of the apparatus is increased due to an additional device for moving the marking back and forth.
Further, there may be another method in which a marking is placed onto a document pressing plate or a reference white plate. However, there is a gap generated in the depth direction between the placed marking position and a document surface reading position, therefore, there may arise a problem of causing displacement due to the gap and the like. Further, a sharp line may be used as the marking, however, there may arise a problem; for example, foreign material adhered to the contact glass near the marking position may be mistakenly detected as the marking.